Zero-order diffractive filters (ZOF) are used in different applications. These filters are based on the resonant reflection of a leaky waveguide. The documents related to colour filters, especially to security devices based on ZOF, are discussed in the following paragraphs.
In U.S. Pat. No. 4,484,797 zero-order diffractive filters are described for use as authenticating or security devices. Illuminated even with non-polarized, polychromatic light such devices show unique colour effects upon rotation and therefore can be clearly identified. ZOF with no colour effect upon rotation are not described.
The WO 03/059643 also describes very similar ZOF for use in security elements. The elements have the same drawbacks as the filters in the U.S. Pat. No. 4,484,797.
Security devices with at least two stacked ZOF with a spacing between the filters that allows interference to take place are described in WO2006/038120. Such ZOF show stronger colour effects compared to ZOF mentioned before. Again, ZOF with no colour effect upon rotation are not described.
Other well known optical filter techniques are based on multilayer interference effects. Flat dielectric layer stacks or Fabry-Perot type layer stacks (interference filters) which are disclosed in U.S. Pat. No. 3,858,977 are the most prominent examples. For a strong colour effect an interference layer stack of at least three layers is needed.